👉 The term "underfilling" in the context of nuclear physics refers to a situation where there is an excess of neutrons within a nucleus, which can lead to instability and the formation of unstable nuclei. This can occur due to various factors such as radioactive decay, chemical reactions, or changes in the atomic structure of the nucleus itself. Underfilling can have serious consequences for the stability of nuclear matter, leading to problems with the stability and performance of nuclear devices like nuclear reactors and nuclear weapons.